During a procedure that uses multiple imaging modalities, for example CT and live X-ray, a physician may need to register the modalities. For example, when using an interventional X-ray device, in particular a C-arm device, the physician may move the C-arm to a certain angulation that gives him a clear view on landmarks in a body of a patient that can be used for registration, for example the pelvic bone. He will then acquire an exposure run or a single shot image. Next, the physician may move the C-arm to a different angulation that may be sufficiently different (e.g. more than 45 degrees difference) and shoot another exposure or single shot.
Both X-ray runs may be shown in different viewports on a display device, where they are blended or overlaid with projection images of the CT volume (3D) image. The physician may move the CT image in both views such that it optimally overlaps the live X-ray images. The visual markers that the physician uses to determine optimal overlap are usually bony landmarks, such as the pelvis, spine or thorax. While the pelvic bones are relatively easy to distinguish in an X-ray image, the spine and thorax may be more difficult, especially with obese patients or low-dose X-ray protocols.